Yea, but what about adding this methyl group to the nitrogen of an amine
That is what I referred to: If you add it like this, methyl halide will simply add once more to form tertiary amine and then once again to form a salt.
The most reliable ways of preparing (secondary) methylamine of sort R-NH-Me are:
a) acylation with acetyl chloride and reduction of the obtained amide with NaBH4 or AlLiH4.
b) Schiff condensation with acetaldehyde to form imine followed by its immediate reduction by NaBH4, NaBH3CN, NaBH(OAc)3 etc which is collectively referred to as "reductive amination".
And somebody before posted those two methods, but in my view those are most straightforward and simplest.